Such a process is known from WO-A-02/26690 which discloses the carbonylation of a conjugated diene by reacting said conjugated diene with carbon monoxide and a hydroxyl group-containing compound in the presence of a catalyst system based on (a) a source of palladium cations, (b) certain bidentate diphosphine ligands, and (c) a source of anions. The latter component includes carboxylic acids such as pivalic acid, and monoesters of dicarboxylic acids such as monomethyladipate. According to the description typical bidentate ligands include 9-phosphabicycloalkylalkanes, such as 1,2-bis (9-phosphabicyclononyl)propane, 2,3-bis(9-phosphabicyclononyl)butane, and 2,3-bis(9-phosphabiscyclononyl) pentane, and the like.
It is further stated in said WO-A-02/26690that the carbonylation process of conjugated dienes when carried out with such a specific choice of diphosphine ligand exhibits unexpected advantages with regard to the catalyst activity and, in addition, that the catalyst system remains stable over a prolonged period of time and can be reused several times without loss or substantial loss of catalyst activity. As described, the process is preferably carried out with a slight molar excess of ligand to palladium because this results in that the ligand degradation is decreased or even no ligand degradation occurs. Most preferably, the bidentate ligand/palladium molar ratio is higher than 1.05 and less than 1.2. When performing the process with a slight excess of ligand to palladium, it is preferred to monitor the concentration (and degradation) of the ligand and to add fresh ligand in order to remain in the preferred ranges of operation.
Despite the advantages and progress provided by the process and palladium catalyst system disclosed in the said WO-A-02/26690, the Applicant has found that the catalyst system and especially the ligand which is present in excess to Pd has only a moderate stability.
An object of the present invention is to provide an improved process in terms of stability of the catalyst system.
Without wishing to be bound to any particular theory, applicant has found that the bidentate diphosphine ligands which were applied in WO-A-0226690, such as the above-mentioned bis(9-phosphabicycloalkyl)alkanes but also other mono-, bi- and multidentate phosphine ligands containing at least one phosphorus atom which is directly bound to two or three aliphatic carbon atoms in general, hereinafter collectively referred to as the process ligand, tend to react with for example 2-pentenoic acid derivatives and/or with butadiene and/or polybutadienes in the presence of carboxylic acids under the usual carbonylation conditions, resulting in phosphonium salts. It is believed that at least a portion of the resulting phosphonium salts is not reversible to the starting materials and decomposes to three-valent phosphorus derivatives other than the process ligand. The decomposition products thus formed give also phosphonium salts and decompose even further.
It was also found that the equilibrium between the phosphonium salts and the free ligands is shifted strongly towards the phosphonium salts. Thus, the formation of phosphonium salts resulting from the use of bis(9-phosphabicycloalkyl)alkane or other mono-, bi- and multidentate phosphine ligands containing at least one phosphorus atom which is directly bound to two or three aliphatic carbon atoms, as process ligand seems to represent a degradation path for the ligand resulting in a complete consumption of the available free ligand at some time under carbonylation conditions.
It was also found that adding fresh process ligand under the carbonylation conditions usually applied results in the instantaneous formation of phosphonium salts. Thus, the freshly added ligand is not or only partly available for the regeneration of the catalyst from sub-stoichiometric ligand-Pd species.
It has surprisingly been found that the formation of the above-mentioned phosphonium salts of bis(9-phosphabicycloalkyl)alkanes or other mono-, bi- and multidentate phosphine ligands containing at least one phosphorus atom which is directly bound to two or three aliphatic carbon atoms is relatively slow at 50° C. and negligible at room temperature. It has also surprisingly been found that the regeneration of the Pd catalyst from sub-stoichiometric ligand-Pd species in the presence of an excess amount of the free (fresh) process ligand is very fast at a temperature less than or equal to 50° C.